Multilayer thick film circuits have been used for many years to increase circuit functionality per unit area. Conventionally thick film materials are deposited onto substrates by printing through screens having a desired pattern. One major problem associated with patterning thick film in multilayer configuration is the limited resolution capability by screen printing. In addition, this multilayer approach requires excessive printing and firing steps which adversely affects the mass-productivity and is more expensive. Repeating the printing and firing steps limits the resolution of patterns, and also requires stringent control of thickness for each layer, realignment between the layers and careful processes to avoid blistering and cracking.
A diffusion patterning process as disclosed by Felton in U.S. Pat. No. 5,032,216 can overcome these drawbacks.
In this method, a first layer of non photosensitive polymeric material is applied to a substrate in an unpatterned manner, followed by a second non photosensitive layer with a pattern. Agent(s) from the second patterning layer diffuses downward into the first layer effecting a change in the dispersibility of that layer. The second layer and those areas having greater dispersibility in the first layer by the above action are then removed by washing with a predetermined eluant. For electronic applications, the functional material is contained in the first layer, which is fired after the washing step. As screen printing is pertinent to this invention and has been one of the most commonly used thick film application method, a brief summary of which is provided below.
The screen is one of the most important part of the screen printing equipment. It is responsible for the definition of the printed pattern and is also the major factor controlling the thickness of thick film paste deposited on the substrate surface. During the printing process the squeegee forces paste through the open areas of the screen and at the same time presses the screen into close contact with the substrate, so forming a seal which minimizes sideways spread of the paste between the face of the screen and the substrate.
There are currently two main types of screens available, the conventional mesh based screen and the etched foil stencil. In the later type a sheet of metal foil, such as stainless steel, copper, bronze, or nickel is stretched across a rigid metal frame and selectively etched to produce the required opening pattern. However, the majority of screens used in microelectronic printing are of the mesh type, the basis of which is tightly woven mesh fixed to a rigid metal frame, the openings of the mesh being selectively covered to form a pattern. The patterns are produced by coating the mesh with photosensitive emulsions or films, followed by photo imaging process. Alternatively, etched metal foils may also be attached to the face of the screen. Photosensitive coating materials fall into three main groups: emulsion, which is a viscous fluid; direct film, which is a photosensitive film attached to the mesh prior to pattern development; and indirect film on which the pattern is formed before the film is fixed to the mesh.
For simplicity, this invention uses "emulsion opening" or "aperture" to represent the patterned open area on screens or stencils, regardless of materials or methods of their fabrication.
While diffusion patterning is versatile, fast and economical, it does have some limitation. In particular, like any conventional thick film pastes, a diffusing patternable material also tend to spread in the X-Y direction (lateral spreading) after screen printing. Such lateral spreading produces an enlarged circular deposition of material from a screen circular opening, but an enlarged plus semi-rounded deposition from a straight edge opening on screen. FIG. 1a shows that onto a substrate 1, an unpatterned layer 2 was printed, over which an imaging layer of width 3b was printed from a screen of opening size 3a. Noting that 3b&gt;3a illustrates the lateral spreading. In addition, during a subsequent drying step, the agent contained in the second, imaging layer 3 also diffused in the X-Y direction (the so-called lateral diffusion), as it progressively diffused downward into the unpatterned layer 2 in the Z direction (vertical diffusion). This leads to, after the final washing step, an enlarged patterned features 3c versus 3b, illustrating the effect of lateral diffusion. Both effects makes it particularly difficult to produce via holes, cavities or channels where straight edge and/or high aspect ratio are required.
This invention utilizes both lateral spreading and diffusion effects to design improved emulsion opening geometry which can be used to form screens or stencils photographically, chemically, mechanically, or methods otherwise, and by screen printing to create precisely controlled patterns of a thick film paste as an imaging layer and produce desirable patterned features by completing the diffusion patterning process. FIG. 2 illustrates schematically one emulsion opening, in which part of the wire mesh 5 is covered with emulsion layer 6 to give an opening 7.